1. Field of the Invention
The present invention relates to a light emitting device including a element (hereinafter referred to as an EL element) in which a thin film (hereinafter referred to as an EL film) made of a luminescent material capable of generating electroluminescence (hereinafter abbreviated as EL) is interposed between a pair of electrodes. Incidentally, in the present specification, an EL element using a luminescent material in which EL is obtained by singlet excitation is called a singlet EL element, and an EL element using a luminescent material in which EL is obtained by triplet excitation is called a triplet EL element.
2. Description of the Related Art
In recent years, an EL element using an organic EL film as a light emitting layer has been developed, and EL elements using various organic EL films have been proposed. An attempt to realize a flat panel display by employing a light emitting device, which uses an EL element of the kind as a light emitting element, has been made.
As a light emitting device using an EL element, a passive matrix type and an active matrix type are known. The passive matrix type is a light emitting device using an EL element made of a structure in which stripe-like anodes and cathodes are provided to intersect with each other at right angles and an EL film is interposed between them. The active matrix type is a system in which a semiconductor component is provided for each of pixels, and one of an anode and a cathode of an EL element is connected to the semiconductor component, so that a current flowing through the EL element is controlled by the semiconductor component.
However, in both the passive matrix type light emitting device and the active matrix type light emitting device, since the light emission performance of the EL element is greatly influenced by the physical properties of the EL film itself, the development of the EL element being bright and having high reliability has been just the development of a luminescent material.
Although various kinds of luminescent materials from a low molecular material to a high molecular material are developed, a theoretical upper limit of luminous efficiency has been always a problem. Especially, with respect to internal quantum efficiency, it has been thought that the ratio of the generation efficiency of singlet exciton to the generation efficiency of triplet exciton is 1:3, and only the singlet exciton contributes to light emission (fluorescent emission).
Thus, even if all carriers (electrons and holes) are recombined, 25% of the whole contributes to the light emission, and if extracting efficiency to the outside of the component is 20%, external quantum efficiency becomes 5% in total. That is, according to the calculation, only 5% of consumed energy can be extracted.
However, recently, a material in which light emission (phosphorescent emission) using triplet exciton is proposed, and its high luminous efficiency attracts attention. As examples which the triplet exciton is used and the external quantum efficiency is improved, there are following reports.
(1) T. Tsutsui, C Adachi, S. Saito, Photochemical Processes in Organized Molecular System, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437.
(2) M. A. Baldo, D. F. O""Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 395 (1998) p. 151.
(3) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75 (1999) p.4.
(4) T. Tsutsui, M. -J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38 (12B) (1999) L1502.
The luminescent materials set forth in the above papers are examples in which the external quantum efficiency is improved by obtaining phosphorescence from the triplet exciton.
An object of the present invention is to provide a light emitting device which is bright and has low electric power consumption, by using an EL element having high luminous efficiency.
Another object of the present invention is to provide an electrical appliance which has a bright display portion and low electric power consumption, by using the light emitting device.
Still another object of the present invention is to provide an electrical appliance having low electric power consumption by using the light emitting device of the present invention as a light source (typically, a backlight).
A light emitting device of the present invention is characterized in that a triplet EL element is electrically connected to a semiconductor component and is controlled. That is, the present invention is characterized by using the triplet EL element as a light emitting element in an active matrix type light emitting device. As the semiconductor component, a field effect transistor (FET), preferably a thin film transistor (TFT) can be used.
First, a process up to the present invention will be described. When a voltage is applied between an anode and a cathode of an EL element, a carrier (electron or hole) is injected into an EL film, and luminescence is generated by recombination. Thus, a proportionality relation is obtained between density of current flowing through the EL element and luminous brightness. Incidentally, in the present specification, a voltage applied to an EL element is called an operation voltage of the EL element.
FIG. 3 is a graph schematically showing this relation. FIG. 3 shows the relation between the density of current flowing through an EL element and the luminous brightness. Reference numeral 301 indicates a characteristic of a conventional EL element using singlet excitation (singlet EL element); and 302, a characteristic of an EL element using triplet excitation (triplet EL element) .
The characteristic of the singlet EL element shown as 301 is that although the proportionality relation (linear relation) is obtained when the current density is low, an inclination becomes small as the current density becomes high. That is, it is known that even if the current density increases, the luminous brightness becomes hard to increase over a certain point. This tendency is remarkable in the case of the characteristic of the triplet EL element shown as 302. When the current density is low, it is in the proportionality relation having an inclination larger than the singlet EL element with respect to the luminous brightness. However, when the current density increases, the inclination becomes extremely small, and there occurs such a state where luminous brightness is hardly changed even if the current density increases.
From the graph of FIG. 3, it is understood that although the luminous brightness is several times higher than that of the singlet EL element when the triplet EL element emits light in the operation region of small current density, it becomes almost equal to the luminous brightness of the singlet EL element in the operation region of high current density.
FIG. 4 shows a relation between the operation voltage of an EL element and its luminous efficiency. Reference numeral 401 indicates a characteristic of a singlet EL element; and 402, a characteristic of a triplet EL element. An operation voltage xe2x80x9caxe2x80x9d indicates an operation voltage (8 to 12 V) where the luminous efficiency of the singlet EL element becomes highest, and an operation voltage xe2x80x9cbxe2x80x9d indicates an operation voltage (3 to 5 V) where the luminous efficiency of the triplet EL element becomes highest.
At this time, as shown in the graph of FIG. 4, the triplet EL element has a feature that its luminous efficiency becomes highest when the operation voltage is lower than the singlet EL element. That is, the triplet EL element shows higher luminous efficiency than the singlet EL element in the operation region of low current density. This is not contradictory to the graph of FIG. 3 in which the triplet EL element shows higher luminous brightness than the singlet EL element in the operation region of low current density.
Here, from the characteristics of the triplet EL element shown in the graphs of FIGS. 3 and 4, relations in FIGS. 5 and 6 can be schematically derived. A graph of FIG. 5 shows a relation between electric power consumption and luminous efficiency in the triplet EL element, and it is understood that the luminous efficiency becomes low in the operation region of high electric power consumption. A graph of FIG. 6 shows a relation between electric power consumption and luminous brightness in the triplet EL element, and it is understood that an increasing rate of the luminous brightness is low in the operation region of high electric power consumption.
From the above, the present inventor has considered that it is desirable to make the triplet EL element emit light in the operation region of low current density. that is, in the operation region of low operation voltage.
Here, the driving principle of a passive matrix type light emitting device becomes a problem. In the case of the passive matrix type light emitting device, since only one selected pixel emits light, a time obtained by dividing one frame period (normally {fraction (1/60)} second) by the number of pixels becomes a luminous time. That is, as the number of pixels becomes large and the fineness becomes high, a luminous time per pixel becomes short. Thus, in order to enable a bright and highly fine image display, the luminous brightness per pixel is raised, and a high density current must be made to flow instantaneously.
Accordingly, in the case where the triplet EL element is used for the passive matrix type light emitting device, light emission is made in the operation region of high current density in FIG. 3, that is, by the operation voltage higher than the operation voltage xe2x80x9cbxe2x80x9d in FIG. 4, and the light is made to emit in the operation region where the luminous efficiency is low. Accordingly, in order to obtain high luminous brightness, it is necessary that a higher current is made to flow, and as a result, the electric power consumption increases and the EL film is deteriorated.
From the above, the present inventor has considered that the passive matrix type light emitting device is disadvantageous to the light emission of the triplet EL element, and has found that an active matrix type light emitting device is most suitable for the triplet EL element. This is because a luminous time can be controlled by a semiconductor component in the active matrix type light emitting device, so that the density of current made to flow through the triplet EL element can be greatly suppressed.
Therefore, the inventor considers that the inventiveness of the present invention is in the point that the triplet EL element is combined with the active matrix type light emitting device as the most suitable light emitting device in view of the electric characteristics of the triplet EL element.
In the active matrix type light emitting device using the triplet EL element according to the present invention, when the operation voltage of the EL element is low, high luminous efficiency is obtained, so that it becomes possible to carry out a bright image display having high luminous brightness. Accordingly, the operation voltage may be made 10 V or less, preferably 7.5 V or less, more preferably 5 V or less. Besides, since it is expected that materials are further developed from now, it is conceivable that the operation voltage becomes 2.5 to 10 V.
Besides, in the active matrix type light emitting device, since the luminous time of the EL element can be made long, the operation voltage of the EL element can be set low when the same brightness (illuminance) as the passive matrix type light emitting device is secured. That is, the electric power consumption can be suppressed as compared with the passive matrix type light emitting device.
This point is schematically shown in FIGS. 7A and 7B. In FIG. 7A, the horizontal axis indicates the number of pixels included in a pixel portion, and the vertical axis indicates the luminous time in an arbitrary pixel. Incidentally, the luminous time in an arbitrary pixel is a time when the arbitrary one pixel continues emitting light, and here, in the case of the active matrix type (indicated by 701) and the case of the passive matrix type (indicated by 702), a time necessary to secure the same luminous brightness is considered as the luminous time.
The graph of FIG. 7A shows that when the number of pixels becomes large (becomes high fineness), the luminous times necessary to secure the same luminous brightness are different from each other between the passive matrix type and the active matrix type. That is, since the luminous time can be controlled by a semiconductor component in the active matrix type, the almost equal luminous time can be secured irrespective of the number of pixels. However, in the passive matrix type, when the number of pixels increases, the luminous time decreases.
Thus, in order to secure the same luminous brightness, the relation between the number of pixels and the current density becomes as shown in FIG. 7B. That is, in the case of the active matrix type (indicated by 703), even if the number of pixels becomes large, the current density may be almost constant and small. However, in the case of the passive matrix type (indicated by 704), when the number of pixels becomes large, the current density necessary to secure the luminous brightness greatly increases.
Therefore, when the number of pixels increases and high fineness is achieved, the active matrix type light emitting device in which the current density may be low, becomes advantageous in view of the suppression of electric power consumption as compared with the passive matrix type light emitting device.
Therefore, since the triplet EL element can obtain the most efficient light emission in the region of low operation voltage, when it is combined with the active matrix type light emitting device, it is possible to realize a light emitting device which has low electric power consumption and enables a bright image display. Further, since the operation voltage of the EL element is low, the density of current made to flow through the EL element may be low, so that the light emitting device having a long lifetime (high reliability) can be obtained.